This invention relates to the field of cathodic protection systems and method and in particular for determining the true cathode polarization potential in the system.
The use of cathodic protection systems to protect a cathode metal in contact with an electrolyte fluid is well known. Generally, cathodic protection systems are of two types -- sacrificial anode or impressed current.
The sacrificial anode relies upon the natural difference in electrical potential between a cathode and an anode to sacrifice or consume the anode to protect the cathode. As such systems rely upon the natural difference in potential there is no need to measure and compensate for changes in the electrical potential between the anode and cathode.
The latter type -- impressed current -- usually relies upon a rectifier to supply an impressed electrical direct current between the anode and cathode, but other sources of direct current may be used. For examples, see Control of Pipeline Corrosion by A. W. Peabody, copyrighted 1967 by, and available from, the National Association of Corrosion Engineers, 2400 West Loop South, Houston, Tex., 77027.
In general, the direct current impressed current producing rectifiers are powered by either 3-phase or single-phase alternating current (hereinafter AC) that is usually reduced in voltage by a transformer before being rectified into a direct current (hereinafter DC) output of a desired type. Normally, electrical current rectification is done by either a selenium or silicon rectifying disc or diode to attain the overall DC voltage output desired.
Impressed current systems may also be used to protect an anodic pasivation system such as disclosed in Bank, et al U.S. Pat. Nos. 3,378,472; 3,375,183; and 3,371,023.
Precise control of the impressed current in a cathodic protection system is not only highly desirable, but a prime requirement. Early impressed current cathodic protection systems, for instance that disclosed in U.S. Pat. No. 2,176,514, lacked means for adjusting the impressed current to a changing environment. If the impressed current used was less than that required by the system, undesired corrosion of the cathode resulted. If on the other hand, the impressed current used exceeded the system requirements electrical power was wasted and paint "blistering" or other damage to the cathode's protective coating results.
Earlier attempts to solve those problems used precise electrical output apparatus such as that disclosed in U.S. Pat. Nos. 2,332,955; 2,584,816, and 2,368,264.
However, it was quickly recognized that the cathodic protection system reference or natural voltage was varied by a number of factors, such as the metal to be protected and the environmental conditions and which changed from time to time. To compensate for such changes the rectifier output of direct current was made adjustable. Some were manually adjustable as disclosed in Polin U.S. Pat. No. 2,021,519. Examples of automatically adjusting cathodic protection systems are disclosed in U.S. Pat. Nos. 1,891,005; 2,759,887; and 3,143,670, while U.S. Pat. Nos. 1,142,858 and 1,438,946 disclose general purpose output self-adjusting electrical apparatus. This automatic control or adjustment has usually been achieved in the prior art using saturable reactor control or with a silicon controlled rectifier (hereinafter SCR). For additional information, see the August 1968 article by one of the inventors of the present invention at pages 26-29 of Materials Protection, available from the National Association of Corrosion Engineers at the above address.
In U.S. Pat. Nos. 2,986,512; 2,982,714; 2,987,461; and 2,998,371, all to Sabins, there is disclosed a number of control systems for automatically controlling the impressed current rectifier output. Another example employing transistors may be found in Andersen, et al, U.S. Pat. No. 3,953,742 or Rubelman U.S. Pat. No. 3,373,100.
U.S. Pat. No. 3,129,154 discloses a compensating method of controlling the impressed current in which a known electromotive force is made opposed to the unknown reference electromotive force. In such arrangement, the electrical current flow through the reference circuit is minimized and polarization of the reference electrode, as well as the resulting deterioration, is minimized.
U.S. Pat. No. 3,634,222 disclosed an improved cathodic protection automatic control system in which the true cathode polarization potential could be determined using the "instant off" method. While such system does eliminate some of the error in determining the true potential, the system employed a sequential controller that was subject to failure and periodically interrupted the operation of the cathodic protection of the system.